1,199 research outputs found
The spectral variability and magnetic field characteristics of the Of?p star HD 148937
We report magnetic and spectroscopic observations and modeling of the Of?p
star HD 148937 within the context of the MiMeS LP at the CFHT. Thirty-two high
signal-to-noise ratio circularly polarised (Stokes V) spectra and 13
unpolarised (Stokes I) spectra of HD 148937 were acquired in 2009 and 2010. A
definite detection of a Stokes V Zeeman signature is obtained in the grand mean
of all observations (in both LSD mean profiles and individual spectral lines).
The longitudinal magnetic field inferred from the Stokes V LSD profiles is
consistently negative, in contrast to the essentially zero field strength
measured from the diagnostic null profiles. A period search of equivalent width
measurements confirms the previously-reported 7.03 d variability period. The
variation of equivalent widths is not strictly periodic: we present evidence
for evolution of the amount or distribution of circumstellar plasma.
Interpreting the 7.03 d period as the stellar rotational period within the
context of the ORM, we have phased the equivalent widths and longitudinal field
measurements. The longitudinal field measurements show a weak sinusoidal
variation of constant sign, with extrema out of phase with the H{\alpha}
variation by about 0.25 cycles. The inferred magnetic configuration confirms
the suggestion of Naz\'e et al (2010), who proposed that the weaker variability
of HD 148937 as compared to other members of this class is a consequence of the
stellar geometry. Based on the derived magnetic properties and published wind
characteristics, we find a wind magnetic confinement parameter \eta\ast \simeq
20 and rotation parameter W = 0.12, supporting a picture in which the Halpha
emission and other line variability have their origin in an oblique, rigidly
rotating magnetospheric structure resulting from a magnetically channeled wind.
(Abridged.)Comment: 13 pages, MNRAS. Version 2, small change to Fig. 1
Lung function decline over 25 years of follow-up among black and white adults in the ARIC study cohort
Interpretation of longitudinal information about lung function decline from middle to older age has been limited by loss to follow-up that may be correlated with baseline lung function or the rate of decline. We conducted these analyses to estimate age-related decline in lung function across groups of race, sex, and smoking status while accounting for dropout from the Atherosclerosis Risk in Communities Study
Field linkage and magnetic helicity density
The helicity of a magnetic field is a fundamental property that is conserved
in ideal MHD. It can be explored in the stellar context by mapping large-scale
magnetic fields across stellar surfaces using Zeeman-Doppler imaging. A recent
study of 51 stars in the mass range 0.1-1.34 M showed that the
photospheric magnetic helicity density follows a single power law when plotted
against the toroidal field energy, but splits into two branches when plotted
against the poloidal field energy. These two branches divide stars above and
below 0.5 M. We present here a novel method of visualising the
helicity density in terms of the linkage of the toroidal and poloidal fields
that are mapped across the stellar surface. This approach allows us to classify
the field linkages that provide the helicity density for stars of different
masses and rotation rates. We find that stars on the lower-mass branch tend to
have toroidal fields that are non-axisymmetric and so link through regions of
positive and negative poloidal field. A lower-mass star may have the same
helicity density as a higher-mass star, despite having a stronger poloidal
field. Lower-mass stars are therefore less efficient at generating large-scale
helicity.Comment: 8 pages, 7 figures, 1 tabl
The winds of young Solar-type stars in Coma Berenices and Hercules-Lyra
We present wind models of 10 young Solar-type stars in the Hercules-Lyra association and the Coma Berenices cluster aged around ∼0.26 and ∼0.58 Gyr, respectively. Combined with five previously modelled stars in the Hyades cluster, aged ∼0.63 Gyr, we obtain a large atlas of 15 observationally based wind models. We find varied geometries, multi-armed structures in the equatorial plane, and a greater spread in quantities such as the angular momentum loss. In our models, we infer variation of a factor of ∼6 in wind angular momentum loss J˙ and a factor of ∼2 in wind mass-loss M˙ based on magnetic field geometry differences when adjusting for the unsigned surface magnetic flux. We observe a large variation factor of ∼4 in wind pressure for an Earth-like planet; we attribute this to variations in the ‘magnetic inclination’ of the magnetic dipole axis with respect to the stellar axis of rotation. Within our models, we observe a tight correlation between unsigned open magnetic flux and angular momentum loss. To account for possible underreporting of the observed magnetic field strength we investigate a second series of wind models where the magnetic field has been scaled by a factor of 5. This gives M˙∝B0.4 and J˙∝B1.0 as a result of pure magnetic scaling
Biomarkers and degree of atherosclerosis are independently associated with incident atherosclerotic cardiovascular disease in a primary prevention cohort: The ARIC study
Biomarkers and atherosclerosis imaging have been studied individually for association with incident cardiovascular disease (CVD); however, limited data exist on whether the biomarkers are associated with events with a similar magnitude in the presence of atherosclerosis. In this study, we assessed whether the presence of atherosclerosis as measured by carotid intima media thickness (cIMT) affects the association between biomarkers known to be associated with coronary heart disease (CHD) and incident cardiovascular disease (CVD) in a primary prevention cohort
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The variable magnetic field of V889 Her and the challenge of detecting exoplanets around young Suns using Gaussian process regression
Discovering exoplanets orbiting young Suns can provide insight into the formation and early evolution of our own solar system, but the extreme magnetic activity of young stars obfuscates exoplanet detection. Here we monitor the long-term magnetic field and chromospheric activity variability of the young solar analogue V889 Her, model the activity-induced radial velocity variations and evaluate the impacts of extreme magnetism on exoplanet detection thresholds. We map the magnetic field and surface brightness for 14 epochs between 2004 and 2019. Our results show potential 3-4 yr variations of the magnetic field which evolves from weak and simple during chromospheric activity minima to strong and complex during activity maxima but without any polarity reversals. A persistent, temporally-varying polar spot coexists with weaker, short-lived lower-latitude spots. Due to their different decay time-scales, significant differential rotation and the limited temporal coverage of our legacy data, we were unable to reliably model the activity-induced radial velocity using Gaussian Process regression. Doppler Imaging can be a useful method for modelling the magnetic activity jitter of extremely active stars using data with large phase gaps. Given our data and using Doppler Imaging to filter activity jitter, we estimate that we could detect Jupiter-mass planets with orbital periods of ∼3 d. A longer baseline of continuous observations is the best observing strategy for the detection of exoplanets orbiting highly active stars
Discovery of a weak magnetic field in the photosphere of the single giant Pollux
Aims: We observe the nearby, weakly-active single giant, Pollux, in order to
directly study and infer the nature of its magnetic field. Methods: We used the
new generation spectropolarimeters ESPaDOnS and NARVAL to observe and detect
circular polarization within the photospheric absorption lines of Pollux. Our
observations span 18 months from 2007-2009. We treated the spectropolarimetric
data using the Least-Squares Deconvolution method to create high
signal-to-noise ratio mean Stokes V profiles. We also measured the classical
activity indicator S-index for the Ca H&K lines, and the stellar radial
velocity (RV). Results: We have unambiguously detected a weak Stokes V signal
in the spectral lines of Pollux, and measured the related surface-averaged
longitudinal magnetic field Bl. The longitudinal field averaged over the span
of the observations is below one gauss. Our data suggest variations of the
longitudinal magnetic field, but no significant variation of the S-index. We
observe variations of RV which are qualitatively consistent with the published
ephemeris for a proposed exoplanet orbiting Pollux. The observed variations of
Bl appear to mimic those of RV, but additional data for this relationship to be
established. Using evolutionary models including the effects of rotation, we
derive the mass of Pollux and we discuss its evolutionary status and the origin
of its magnetic field. Conclusions: This work presents the first direct
detection of the magnetic field of Pollux, and demonstrates that ESPaDOnS and
NARVAL are capable of obtaining sub-G measurements of the surface-averaged
longitudinal magnetic field of giant stars, and of directly studying the
relationships between magnetic activity, stellar evolution and planet hosting
of these stars.Comment: 8 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
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